Causes and Countermeasures of Silica Contamination in Reverse Osmosis Operations

Silica contamination in reverse osmosis systems is a thorny issue in Water Treatment, with complex causes and high remediation costs.

I. Causes of Silica Contamination

1. Supersaturation and Precipitation of Dissolved Silica

The concentration of active silica (H₄SiO₄) in source water increases dramatically during the Ro Membrane concentration process. When the concentration factor exceeds the solubility limit of silica (typically 50-150 mg/L, affected by pH and temperature), silicic acid polymerizes to form insoluble amorphous silica colloids, which adhere to the membrane surface.

2. Direct Deposition of Colloidal Silica

Colloidal silica (particle size 0.01-1μm) in natural water is not completely removed by pretreatment. As it passes through the membrane surface, it is enriched and deposited by concentration polarization, forming a dense gel layer.

3. Co-contamination by Metal Silicates

Metal ions such as aluminum, iron, and calcium combine with silicate ions to form insoluble salts such as aluminum silicate and iron silicate. This type of complex is hard and more difficult to clean, and is commonly found in water sources with high hardness or containing iron and aluminum.

4. Operating Conditions Exacerbate Contamination

High Recovery Operation: The silicon concentration in the final membrane element increases exponentially, increasing the risk of contamination. Low-Temperature Operation: The silicon solubility decreases significantly with decreasing temperature (120 mg/L at 25°C → only 60 mg/L at 5°C).

II. Prevention and Control Measures

1. Water Source Adaptation Pretreatment

Enhanced Colloidal Removal: For colloidal silica, use ultrafiltration (UF) or high-efficiency coagulation (e.g., ferrate) combined with multi-media filtration.

Silicon Form Adjustment: For high-silicon groundwater, use lime softening combined with magnesium desiliconization to reduce dissolved silicon to below 20 mg/L.

Ion Exchange Metal Removal: A weak acid cationic bed removes Ca²⁺/Mg²⁺, reducing the risk of silicate scaling.

2. Optimized RO System Design

Segmented Design: Reduce final recovery and control the silicon concentration on the membrane surface to <90% of the solubility limit.

Anti-fouling membrane selection: Choose an anti-fouling membrane with a smooth surface and high hydrophilicity (such as a polyamide composite membrane).

Temperature management: Install a heater on the low-temperature water source to maintain a water temperature >15°C.

3. Intelligent operating parameters

Install online monitoring of silicon concentration to dynamically adjust the recovery rate. Control the inlet water pH between 6 and 7.5 (the range where silicon solubility is high). Use a variable frequency pump to achieve smooth flow switching and reduce shock loads.

4. High-efficiency scale inhibition solution

Use a specialized scale inhibitor (such as one containing polyvinylphosphonic acid) to effectively delay silicon polymerization. Use software simulation (such as WinFlows) to accurately calculate the dosage to avoid overdosing or underdosing.